1. Field of the Invention
The present invention relates to a vane pump suitable for use in a power steering system.
2. Description of the Prior Art
Recently, power steering systems for motor vehicles tend to use a pressure balance type vane pump having eight vanes in place of those having twelve or ten vanes. Vane pumps with eight vanes are advatageous in that they are lightweight and easy to machine because the number of vanes is small, although they are liable to suffer from variation in discharge volume due to various causes and to generate pressure pulsation caused by the variation in discharge volume. The generation of pressure pulsation is attributed mainly to the following two causes. The first is the variation in theoretical discharge volume which is geometrically calculated based upon the shapes of a cam ring, vanes and the like, and the second is the variation in volume of fluid leakage inside the pump, that is, the variation in volume of leakage depending upon the pump stages within which pressurized fluid leakage occurs.
It is to be noted herein that the aforementioned variation in theoretical discharge volume constitutes an amplitude variation which coincides with the difference between the maximum and minimum values on a curve which indicates discharge volumes at respective angular positions of a pump rotor. It is also to be noted that the value (i.e., the absolute value of the discharge volume) which is obtained by integrating values on the volume curve has no relation to pulsation, although it influences the pump efficiency.
Generally, a cam curve along which the vanes are moved is composed of an intake curve section C1, a large circular section C2, an exhaust curve section C3 and a small circular section C4, as illustrated by means of an expansion plan of FIG. 1. In pumps of this kind, the variation in volume of a chamber is defined by two successive vanes which, respectively, come up to, and go away from an exhaust port OP when a rotor R is moved a unit angle .DELTA..theta. to produce a pump discharge volume. This discharge volume is constant if both the large circular section C2 and the small circrlar section C4 are perfectly circular. However, the large circular section C2 is customarily given a slight gradient for preparatory compression. Accordingly, the discharge volume per unit angle of rotor rotation varies depending upon the preparatory compression gradient and has a discharge volume variation X1 of relatively small amplitude, as shown in FIG. 3. This discharge volume variation is generally called "basic discharge volume variation."
Further, since the vanes V are subjected to fluid pressure which exists in a vane back pressure groove G communicating with the exhaust ports OP, the vanes V which move along each intake port IP are extended radially outwardly when the rotor R is rotated the unit angle .DELTA..theta.. This results in consumption of part of the pump discharge volume corresponding to the variation in volume of vane support slits of the rotor R which support the radial extension of the vanes V. Such consumed volume is in proportion to the degree of outward radial extension of the vanes per the unit angle of roation of the rotor R and corresponds to a velocity curve (A in FIG. 1) relating to a vane moving locus. Assuming now, for example, that a vane V1 is at a position (.alpha.) on the small circular section C4, a preceding vane V2 is along the intake curve section C1 at a position (.alpha.+45.degree.), as shown in FIG. 1. As the rotor R rotates, the vane V2 goes away from the intake curve section C1 before the vane V1 comes to the intake curve section C1. When rotation is further advanced, only the vane V1 resides on the intake curve section C1, and a transition occurs such that the extension movement of the vane V1 is decelerated after reaching a maximum velocity. For this reason, and because any portion of the intake curve section C1 and the exhaust curve section C3 is composed of constant acceleration curve (A) shown in FIG. 1 for reliable movement of each vane, the fluid volume consumed by vane extension movement within the intake area varies depending upon the angular position of the vane V moving along the intake curve section. C1. In addition, the greater the thickness of each vane V, the larger is the amplitude variation.
Accordingly, the variation X2 in the theoretical discharge volume, which is determined by various factors of the cam and the vanes (that is, which is geometrically calculated based upon the shapes of the cam, vanes and the like), is calculated as the difference between the variation of the above-noted basic discharge volume and the variation of the volume consumed by the vane extension movement and is indicated by an amplitude variation curve (A) as shown in FIG. 4. The variation X2 in theoretical discharge volume (A) is one cause contributing to discharge pressure pulsation.
The pressure in each pump sector, a pump sector being defined by two consecutive vanes V, the cam ring C, the rotor R and the side plates (not shown), is periodically changed from an intake pressure to an exhaust pressure. Because the vane back pressure groove G pressure is always the same as the exhaust pressure and because a slight clearance is required between the rotor R and each of the side plates, leakage of pressurized fluid occurs from the vane back pressure groove G toward each sector being under less pressure than the discharge pressure.
Moreover, the pressure balance type pump with eight vanes is accompanied by a problem in that the number of stages where leakage occurs is periodically changed unless the angular positions of the intake and exhaust ports and the angular widths thereof are adequately designed. For example, each exhaust section covers two pump sectors in a state shown in FIG. 1, while it covers three pump sectors in another state shown in FIG. 2. In this manner, the number of pump sectors which isolate each exhaust section from the two intake sections is alternately changed from three to two, and vice versa, each time the rotor R is advanced one vane pitch. Fluid leakage from the vane back pressure groove G takes place within sections other than the exhaust sections. The stage (i.e., angular area) covering such other sections thus periodically varies, and this causes the volume of fluid leakage to vary as indicated by the curve X3 in FIG. 4.
The variation of actual discharge volume of the pump amounts to the difference between the variation X2 in the above-noted theoretical discharge volume (A) and the variation X3 in leakage volume (B). The variation X2 in theoretical discharge volume (A) is determined solely by various factors of the cam and the vanes, while the variation X3 in leakage volume (B) is determined as a function of the pressure difference between the vane back pressure groove G and the intake sections. Accordingly, the variation X3 in amplitude of the leakage volume (B) becomes larger as the load pressure is increased. As a result, when the pump is operated without a load, the pressure difference between the vane back pressure groove G and the intake sections is small, and hence, the influence by the variation X3 in leakage volume (B) is small, so that the variation of actual discharge volume depends greatly upon the variation X2 in theoretical discharge volume (A). When the pressure difference between the vane back pressure groove G and the intake sections become large due to an increase in the pump discharge pressure, however, the variation X3 in leakage volume (B) is much greater than the variation X2 in theoretical discharge volume (A), so that the variation in actual discharge volume depends largely upon the variation X3 of leakage volume (B).
In vane pumps for vehicle power steering systems, because the load pressure varies markedly, it is particularly important to minimize the variation of discharge volume relative to the discharge pressure change.